New flakeheaven rules

Consider the following rules for .flakerules.toml:

[tool.flakeheaven.plugins]
pycodestyle = ["-E701", "-E226", "-W191"]

[tool.flakeheaven.exceptions."tests/"]
flake8-docstrings = ["-D100"]

[tool.flakeheaven.exceptions."README.md"]
flake8-docstrings = ["-*"]

[tool.flakeheaven.exceptions."doc/*"]
flake8-docstrings = ["-*"]